Environmentally driven regulatory pressure concerning motor gasoline sulfur levels are expected to result in the widespread production of less than 50 wppm sulfur mogas by the year 2004. Levels below 10 wppm are being considered for later years. In general, this will require deep desulfurization of cracked naphthas. Cracked naphthas are naphthas resulting from fluidized catalytic cracking operations, steam cracking, coking and related processes. Such naphthas typically contain substantial amounts of both sulfur and olefins. Deep desulfurization of cracked naphthas requires improved technology to reduce sulfur levels without the severe loss of octane that accompanies the undesirable saturation of olefins.
Hydrodesulfurization is one of the fundamental hydrotreating processes of refining and petrochemical industries. Hydrodesulfurization removes organically bound sulfur by conversion to hydrogen sulfide which is typically achieved by reaction with hydrogen over non-noble metal sulfided supported and unsupported catalysts, especially those of Co/Mo and Ni/Mo. This is usually achieved at fairly severe temperatures and pressures in order to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur sensitive process.
Olefinic naphtha, such as cracked naphthas from fluid catalytic cracking and coker naphthas, can contain more than about 20 wt. % olefins. At least a portion of the olefins are hydrogenated during the hydrodesulfurization operation. Since olefins are high-octane components, for some motor fuel use, it is typically desirable to retain as much of the olefins as possible rather than to hydrogenate them to saturated compounds that are typically lower in octane. Conventional fresh hydrodesulfurization catalysts have both hydrogenation and desulfurization activity. Hydrodesulfurization of cracked naphthas using conventional naphtha desulfurization catalysts, under conventional conditions required for sulfur removal, typically leads to a significant loss of olefins through hydrogenation. This results in a lower grade fuel product that needs additional refining, such as isomerization, blending, etc., to produce the desired higher-octane fuels. Such additional refining, or course, adds significantly to the cost of the final naphtha product.
Selective hydrodesulfurization to remove organically bound sulfur while minimizing hydrogenation of olefins and octane reduction by various techniques, such as selective catalysts and/or process conditions, has been described in the art. For example, a process referred to as SCANfining has been developed by ExxonMobil Corporation in which olefinic naphthas are selectively desulfurized with little loss in octane. U.S. Pat. Nos. 5,985,136; 6,013,598; and 6,126,814, all of which are incorporated by reference herein, disclose various aspects of SCANfining. Although selective hydrodesulfurization processes have been developed to avoid significant olefin saturation and loss of octane, such processes have a tendency to liberate H2S that reacts with retained olefins to form mercaptan sulfur by reversion.
Many refiners are considering combinations of available sulfur removal technologies in order to optimize economic objectives. As refiners have sought to minimize capital investment to meet low sulfur mogas objectives, technology providers have devised various strategies that include distillation of the cracked naphtha into various fractions that are best suited to individual sulfur removal technologies. While economics of such systems may appear favorable compared to a single processing technology, the complexity of overall refinery operations is increased and successful mogas production is dependent upon numerous critical sulfur removal operations. Economically competitive sulfur removal strategies that minimize olefin saturation and capital investment and operational complexity are favored by refiners.
Consequently, there is a need in the art for technology that will reduce the cost of hydrotreating cracked naphthas, such as cat cracked naphthas and coker naphthas. There is also a need for more economical hydrotreating processes that minimize olefin saturation, total sulfur, and mercaptan sulfur resulting from mercaptan reversion.